Use Result enum instead of hardcoding "Unknown" username about whoami HOT 1 CLOSED

@kiilerix this is already planned 😄

See #31 (4th item down)

And #30

Exact details of the plan haven't been written down yet, though - so I'll put them here, and use this issue to track completion.

• Step 1; #30
• Step 2; Add fallible module for whoami version 1
• Step 3; Deprecate? All non-fallible APIs
• Step 4; For whoami version 2, remove non-fallible APIs and make the fallible module the root API

If step 2 is completed (which depends on step 1), I think that should resolve this issue, step 3 and 4 are likely future changes I've been thinking about.

I should really add #30 to This Week in Rust's call for participation section...

from whoami.